1. Field of the Invention
The present invention relates to a flat panel display (FPD) and method of fabricating the same and, more particularly, to a FPD and method of fabricating the same, which prevent a pad portion from disconnecting from a driver integrated circuit (IC).
2. Discussion of the Background
Generally, cathode ray tubes (CRT), which are typically used for display devices, are often used in monitors for TVs, measuring instruments, and information terminals. However, due to the CRTs' weight and size, it is difficult to utilize them in small, lightweight electronic products.
Hence, compact and lightweight FPDs have attracted much attention as substitutes for the CRT. FPDs include liquid crystal displays (LCD), organic light-emitting displays (OLED), and the like.
A FPD typically includes a thin film transistor (TFT) substrate including TFTs and red (R), green (G), and blue (B) light emitting diodes (LEDs).
A pad portion, which receives an external signal, is arranged at the periphery of the FPD. The pad portion is connected to a driver IC that generates a scanning or signal voltage. In order to electrically connect the pad portion to the driver IC, a bonding process may be performed using a conductive film, such as an anisotropic conductive film (ACF).
Hereinafter, a conventional OLED will be described with reference to FIG. 1, FIG. 2A, FIG. 2B, FIG. 3A, and FIG. 3B.
FIG. 1 is a cross-sectional view of a conventional OLED. Referring to FIG. 1, the OLED includes an OLED portion 20, a pad portion 30, and an encapsulation substrate 40. The OLED portion 20 is arranged on a transparent insulating substrate 10. The pad portion 30 includes an external circuit module that is connected to the OLED portion 20. The encapsulation substrate 40 is coupled with the transparent insulating substrate 10 using a sealant 50. The OLED portion 20 includes an organic light emitting diode, a TFT, a capacitor, and conductive lines such as a gate line and a data line. The organic light emitting diode includes a first electrode, a second electrode, and an emission layer arranged between the first and second electrodes. In a pixel, the organic light emitting diode is connected to the TFT and the capacitor, and each pixel includes conductive lines such as the gate line and data line. The pad portion 30 electrically connects the OLED portion 20 to an external driver IC.
FIG. 2A is a photograph of a pad portion of a conventional FPD, FIG. 2B is a plan view of a portion “a” of FIG. 2A, and FIG. 3A and FIG. 3B are cross-sectional views taken along line A-A′ of FIG. 2B, which show exemplary process operations.
First, the pad portion is arranged at the periphery of an OLED portion and simultaneously formed with the OLED portion.
While forming source and drain electrodes in a TFT region of a transparent insulating substrate 100, a pad electrode 110 is formed on the pad portion. Here, the pad electrode 110 may be formed of molybdenum (Mo) or molybdenum tungsten (MoW).
Thereafter, a passivation layer 120 is formed on the pad portion when forming a passivation layer on the TFT region.
Subsequently, while forming a via contact hole in the TFT region to expose either the source or drain electrode of the TFTs, a plurality of contact holes 122 is formed in the passivation layer 120 to expose the pad electrode 110 of the pad portion.
A reflective layer 130 is then formed on the entire surface of the resultant structure. The reflective layer 130 may be formed of an aluminum or aluminum alloy layer.
The reflective layer 130 is then selectively removed such that it remains only on an emission region of the OLED portion. Since a contact resistance between the reflective layer 130 and a subsequently formed transparent electrode may increase, it may be desirable to remove the reflective layer 130 from the via contact hole in addition to the pad portion. As FIG. 3B shows, when removing the reflective layer 130, the pad electrode 110 of the pad portion is also removed to a predetermined thickness, thereby forming an undercut 112 under the passivation layer 120.
Next, a transparent metal layer is formed on the entire surface of the resultant structure, followed by photolithography and etching processes, thereby forming a transparent electrode 140 on the emission region of the OLED portion and the pad portion. Here, the transparent electrode 140 formed on the pad portion may be electrically disconnected at the undercut 112 under the passivation layer 120.
The pad portion and a driver IC (not shown) are then connected to a connector, such as a flexible printed circuit (FPC) or a chip on glass (COG). The pad portion may be connected to the connector using an ACF containing conductive particles.
As described above, in a conventional FPD, the pad portion may be directly bonded to the driver IC using a COG process, which provides for smaller devices and lower production cost. However, because the contact holes formed in the pad portion typically have a very fine size of 4 to 10 μm, when the conductive particles with 5 to several tens of μm are not filled in the contact hole, the conductive particles are not connected to the pad electrode formed in the contact hole so that they are electrically disconnected. Also, when the ACF is formed on the transparent electrode disposed on the contact hole, it may be disconnected from the pad electrode due to the undercut formed at the edge of the contact hole. Thus, the OLED portion may not be electrically connected to an external module. Consequently, the contact resistance of the pad portion increases, and the electrical characteristics of the OLED deteriorate.